1. Field of the Invention
The present invention is directed to an arrangement for tissue simulation, in particular a heart pacemaker, which includes a substraction stage for generating a different signal from measured cardiac electrical potential and a curve representing polarization phenomena produced in the tissue by stimulation, with the stimulation being controlled dependent on an evaluation of the difference signal.
2. Description of the Prior Art
A heart pacemaker disclosed in U.S. Pat. No. 4,537,201 with which the heart of a patient is stimulated by electrical stimulation pulses that are generated by a stimulation pulse generator and are supplied to the heart via an electrode arrangement. A contraction ensues, as a reaction of the heart to the stimulation, only when the stimulation energy exceeds a defined stimulation threshold deriving from the stimulation sensitivity of the heart tissue. In order to be able to adapt the stimulation energy to the variable stimulation sensitivity and thus to be able to keep the energy consumption low in the known heart pacemaker, the electrical potential in the heart tissue to be stimulated is acquired by a detector means and is interpreted after every stimulation to see whether a stimulation response, i.e. a reaction of the heart tissue as a consequence of the stimulation, is present. Every stimulation, however, causes polarization phenomena in the heart tissue having a multiply higher amplitude than the potential stimulation response to be detected. In order to enable an identification of the stimulation response in the curve of electrical potential measured by the detector means, the detector means of the known heart pacemaker contains a function generator that generates a signal curve after every stimulation pulse, which describes the polarization phenomena with a logarithmic time function. This signal curve is subtracted from the curve of the electrical potential measured in the heart tissue in a subtraction means and the difference signal obtained in this way is evaluated in an evaluation means for the presence or absence of a stimulation response.
In a further embodiment of the known heart pacemaker, the electrical potential curve acquired by the detector means in the tissue is linearized in an anti-logarithmic amplifier, whereby non-linear deviations from the linearized signal curve are subsequently detected in an evaluation means as a stimulation response of the heart.
In the two embodiments of the known heart pacemaker, the compensation of the polarization phenomena in the acquired curve of potential is based on the assumption that the curve of the polarization phenomena can be described with adequate precision by a defined mathematical equation. Patient-associated and other systemic and non-systemic deviations of the actual curve of the polarization phenomena from the assumed polarization curve, however, are not taken into consideration.
A so-called polarization compensation circuit is provided in another heart pacemaker disclosed in European Application 0 017 848, the polarization components being subtracted therein from the measured, electrical curve of potential of the heart. The difference signal obtained in such a way is evaluated for the presence of QRS waves and T waves. The manner by which the polarization component to be subtracted is obtained, however, is not disclosed in this European Application.